1. Field of the Invention
The present invention relates to the performance of high average power solid-state lasers, and more specifically, it relates to techniques for optically pumping thin solid-state laser media and scaling such laser media to produce high average power.
2. Description of Related Art
The performance of high average power solid-state lasers is in large part determined by the geometry of pump light delivery to the gain medium and on the intensive cooling that must accompany it. For example, some devices utilize pump light impinging on a face of a slab or along the barrel of a rod in a geometry commonly referred to as side-pumping. In some cases, it is desirable to utilize an end-pumping geometry in order to achieve high delivery efficiency due to limitations in the absorption length of the laser material. Side pumping and end pumping schemes rely on side cooling. Side cooling induces stress and optical deformations that limit the quality and ultimately the level of output power possible by either end or side pumping schemes. The subject of this patent adds to the method advanced in the disclosure of the parent application, which described a side pumped laminated light-guide/gain-medium composite that is strikingly robust and resolves prior difficulties with high average power pumping/cooling and the rejection of amplified spontaneous emission.
It is an object of the present invention to provide techniques for scaling the output power of a disk laser to produce high average power.
It is another object of the present invention to provide an apparatus and method that reduce amplified spontaneous emission in the laser gain medium of a thin disk laser.
These and other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.
In this invention, the average power output of a laser is scaled, to first order, by increasing the transverse dimension of the gain medium while increasing the thickness of an index matched light guide proportionately. Strategic facets cut at the edges of the laminated gain medium provide a method by which the pump light introduced through edges of the composite structure is trapped and passes through the gain medium repeatedly. Spontaneous emission escapes the laser volume via these facets. The large face of the laser medium is exposed and is used for cooling. The approach is better described as edge pumping with face cooling. High absorption is possible using moderate concentrations of dopant while minimizing the laser medium thickness.
A key aspect of this invention is the efficient delivery of pump light to a thin-disk laser medium of minimum thickness. This maximizes the laser output because in a face-cooled disk, inversely with the thickness of the gain medium, more average power per unit area is possible for the same peak surface stress.
An optical advantage is also gained from the xe2x80x9cthinnessxe2x80x9d of the laser gain medium, i.e., the fraction of spontaneous emission that remains within the solid angle of the gain medium is proportional to the square of the thickness of the gain medium. Amplified spontaneous emission (ASE) limits the transverse dimensions of an aperture in complex ways related to the geometric details surrounding the gain medium. However, in general, if the detailed geometry is designed properly, a larger transverse aperture is possible with thinner laser medium dimensions.
A thin disk of laser material is bonded to a planar light guide of an index-matched material forming a composite disk. Diode array or other pump light is introduced into the composite disk through the edges of the disk. Pump light trapped within the composite disk depletes as it multi-passes the laser medium before reaching an opposing edge of the disk, thus energizing the laser material. The principles for pumping have been covered in the parent application. The present invention recognizes that in a solid-state laser, the shape of the high index gain medium has an intrinsic effect on the scalability of the output power. The present invention invokes a multi-faceted disk geometry optimized to passively reject spontaneous emission generated within the laser material, otherwise trapped and amplified within the high index composite disk, thus increasing the useful size of the laser aperture, the principal means of scaling the output power. The shape of the disk and details on the strategic placement of facets for the purpose of introducing pump light and/or rejecting spontaneous emission are the subject of this invention. As in the parent application, the resulting compound optical structure delivers pump light efficiently and concentratedly to a laser medium of minimum thickness. The external face of the laser medium is used for cooling. A high performance cooler attached to the external face of the laser medium rejects heat. Laser beam extraction is parallel to the heat flux to minimize optical distortions. Other laser beam extraction schemes are possible.
The present invention has applications in amplified spontaneous emission suppression in solid-state lasers, high average power lasers for industrial applications such as drilling, welding, materials processing, laser illumination, dye laser pumping for isotope separation. Other applications that may become possible with the high brightness afforded include deep laser drilling, thick plate laser welding, large throughput CVD deposition of diamond and other films.
The present invention utilizes all of the embodiments of U.S. patent application Ser. No. 09/237,142, titled xe2x80x9cHigh Average Power Scaleable Thin-Disk Laserxe2x80x9d by Beach et al., filed Jan. 25, 1999 and incorporated herein by reference. Embodiments of lens duct designs usable in the present invention are described in U.S. Pat. No. 5,307,430, which is incorporated herein by reference. Also usable in the invention are optical concentrators, as disclosed in U.S. Pat. No. 6,160,934, titled xe2x80x9cHollow Lensing Ductxe2x80x9d, incorporated herein by reference.